This invention relates to a video signal mixing system, and more particularly to a video signal mixing system in which a blanking interval is formed at a predetermined portion of a video signal, and another video signal is inserted into the formed blanking interval of the video signal.
In some kinds of television receivers, a reduced picture of one channel broadcast is inserted into a picture of another channel broadcast on one screen, or letters or figures are inserted into a picture of one channel broadcast on one screen. The reduced picture, or the letters or figures to be inserted will be called "sub picture" hereinafter. The picture into which the sub picture is inserted will be called "main picture" hereinafter. Video signals for the main picture and sub picture will be called "main video signal" and "sub video signal", hereinafter respectively.
Blanking intervals are formed at predetermined portions of the main video signal. The sub video signal is inserted into the formed blanking intervals. The blanking level is nearly equal to zero. Generally, pedestal levels of the main video signal and sub video signal are different from each other. Accordingly, when the sub video signal is inserted into the blanking intervals of the main video signal, the brightnesses of the main picture and sub picture are different from each other. It is visually preferable that they are equal to each other.
For example, two external variable resistors are provided for removing such difference. In this case, the variable resistors should be individually adjusted. After one of the two variable resistors is adjusted, another of the two variable resistors is adjusted. Or it has been considered to interconnect the two variable resistors so as to remove the above described difference. However, such methods are not preferable.